In reactions in which solid particulate is continuously formed which can concentrate into aggregates it is often desirable to remove such aggregates from the particulate because they are an unwanted product and/or produce fowling of the reactor and/or downstream processing equipment. This is particularly true in a stirred, vapor phase polymerization reactor wherein a small number of dense aggregates formed by local overheating of the polymer particulate matter during operation can cause seizing of the stirrer blades and shut-down of the entire polymerization process. Now a method has been found by which such aggregates or more generally, masses, can be concentrated efficiently towards one end of such reactor to be removed before their number and/or size are large enough to produce process problems.